According to the most recent revision (2004) of the World Health Organization (WHO) histological classification system for lung tumors ("WHO2004"), currently the most widely recognized typing scheme for pulmonary neoplasia, MCACL is considered a distinctive variant of adenocarcinoma.
[3] MCACL has been noted in most cases to show areas of gradual transition wherein cells become more atypical and feature more pronounced characteristics of malignancy as one proceeds from the capsule, or outermost layers of the tumor, toward the center of the mass.
Electron microscopic studies in 3 cases[5][6] described intracytoplasmic mucin, convoluted oval nuclei, prominent nucleoli, homogeneous euchromatin with peripheral chromatin condensation, microvilli, junctional complexes, and primitive lumen formation.
[8] The 1999 World Health Organization classification system defined MCACL as a cystic adenocarcinoma with copious mucin production that, histologically, resembles (the more common) mucus-producing cystadenocarcinomas originating in the ovary, breast and pancreas.
[9] The 2004 revision of the WHO classification noted that the tumors tend to be well circumscribed by a partial fibrous tissue capsule with central cystic change and copious mucin pooling.
[10][11] Microscopically, the neoplastic epithelial cells tend to grow along the alveolar walls, in a fashion similar to the mucinous variant of bronchioloalveolar carcinoma, a more common form of adenocarcinoma.
Iwasaki and co-workers failed to find mutations of the epidermal growth factor receptor (EGFR) or the cellular Kirsten rat sarcoma virus oncogene K-ras in one reported case.